Dumbledore, notorious for giving second chances Dumbledore, let Sirius rot in Azkaban for twelve years.
He must have known Sirius well due to his time in the Order, he must have known what James meant to Sirius. Dumbledore was a member of the freaking Wizengamot yet he didn’t fight the Ministry’s horrifying trial-optional policy.
This is a man who took back Death Eater!Snape at his word, shielded him from prison, and employed him at a school for children.
But he didn’t have a use for Sirius, so he didn’t care about him.
I got 99 problems with Dumbledore and his treatment of Sirius Black accounts for like 64 of them.
To be honest, Albus Dumbledore is one of the most disturbing, terrifying characters I’ve ever found in a book, because he thought he was a good guy and so did everyone else and the books don’t really challenge it either (given that Harry forgives him for everything he did), but when you look between the lines he was profoundly, profoundly immoral and unethical.